Influence of Thunderstorms on Environmental Ozone D. R. DAVIS* National Weather Service Tallahassee, Florida THUNDERSTORMS exert considerable influence upon the environment of many locales. The unique destructive offspring of the thunderstorm which includes lightning, hail, and tornadoes make these storms great destroyers of life and property. Even the rain associated with the thunderstorm often becomes more destructive than beneficial, creating floods which take life and destroy property. These more destructive agents of the thunderstorm have been well publicized, making it unnecessary to do more than mention them. Less known is the fact that there is an indirect, and less dramatic aspect of the thunderstorm which may also have harmful environ­ mental effects. This aspect is the thunderstorm's role as a causal agent in increases of ozone, one of the trace gasses of the atmosphere, in the environment. Thunderstorm induced increases of ozone in the low levels of the atmosphere occasionally are high enough to be harmful to plants and perhaps to animal life. This paper summarizes some of the observed influences of the thunderstorm upon ozone concentration. It also describes the way in which observations indicate that the thunderstorm is able to pro­ duce increases of this gas in the environment. "Present address: Environmental Study Service Center, Auburn University, Auburn, Ala. 36830. 505 D. R. DAVIS INSTRUMENTATION AND METHODS The National Weather Service Office for Agriculture, located at the University of Florida Research and Education Center at Quincy has monitored low-level tropospheric ozone for the past 10 years. Mast Ozone Meters, Model 724-2,1 are operated continuously throughout the year. These instruments use an electrochemical detector to coulometrically measure the oxidation of a buffered po­ tassium iodide solution. Concentrations of ozone are given in parts per hundred million (pphm) by volume and are continuously re­ corded on strip charts. In addition to ozone, observations included precipitation, profile soil and air temperatures, evaporation, humidity, wind direction and velocity including gusts, solar radiation, and sferics (thunderstorm static electricity). The time and type of fronts passing the station as well as airmass type, thunderstorm activity, the occurrence of squall lines and thunderstorm severity, the occurrence of hail, and any other weather information of significance are recorded daily for correlation with ozone concentrations and fluctuations. The ob­ servational record through February, 1973, included approximately 2700 days. The records during thunderstorms were critically examined. When ozone rises were noted, the time of the increases were related to the lightning activity, the beginning of the downdraft winds of the storm, radar reports of storm tops, the height of the tropopause, and any other parameters deemed significant. OZONE Ozone, also referred to as tri-atomic oxygen, is a nearly colorless gaseous form of oxygen. Ozone has a characteristic odor resembling weak chlorine. It is occasionally possible to detect this odor during or following thunderstorms in which there has been a sharp increase in the ambient ozone levels. The normal concentration of this gas is only 1 to 3 pphm by volume in the lower levels of the atmosphere. 1 Mention of trade name and/or manufacturer is for identification only ana does not imply endorsement ·by the Department of Commerce, NO~A, or the National Weather Service. 506 INFLUENCE OF THUNDERSTORMS ON ENVIRONMENTAL OZONE The greatest amount of atmospheric ozone is found above the tropopause in the stratosphere at levels normally above 30,000 feet (Brewer, 1960). Here it is photochemically produced by ultraviolet and near-ultraviolet solar radiation interacting with the oxygen atoms and molecules in the rarified atmosphere. In addition to stratospheric ozone, some ozone is also photochemically produced near the surface of the earth in the lowest layers of the atmosphere. Sunlight inter­ acting with hydrocarbons, nitrogen oxides, sulfur dioxide, and other pollutants is an oxidant producer which includes ozone (Haagen­ Smit, 1952, 1958, Went, 1960a, 1960b). Ozone is a strong oxidant in contact with organic matter and is destroyed rapidly in the oxidation process. Ozone is damaging to both herbaceous and woody plants in relatively small concentrations (Daines et al. Rich, 1964, Davis and Dean, 1966). Damage occurs at even lower concentrations when soil moisture is high (Dean and Davis, 1967). Commercially important forest tree species may ex­ perience needle or leaf damage in concentrations as low as 6.5 pphm (Berry and Ripperton, 1963). Davis arid Dean (1972) made a com­ prehensive review of ozone records in north Florida and related ob­ served values associated with the various weather parameters with the reported damaging threshold concentrations for tree species. The literature has little information on the ozone threshold con­ centrations for damage to insects and other animal life. It is known, however, that ozone is harmful in high concentrations which may reach levels which are lethal even to man. Ten pphm has been es­ tablished as the maximum concentration in which human beings should work and 8 hours is the recommended maximum for one to remain where ozone is this high. Records show a diurnal, day-to-day, seasonal, and annual varia­ tion in the amount of ozone in the troposphere (Davis, 1968, Dean and Davis, 1972). The day -to-day variations are associated with changing weather patterns and phenomena. The thunderstorm, either singularly, in association with cold fronts, as elements of squall lines ahead of cold fronts or thunderstorm conglomerates, is one meteoro­ logical phenomena responsible for some of the increases of ozone in the environment. 507 D. R. DAVIS THUNDERSTORMS The increases of ozone in association with thunderstorms has most frequently been attributed to local production by the lightning stroke. There are two ways in which ozone production by the light­ ning stroke may take place. First, the interaction of the ultraviolet and near-ultraviolet rays of the lightning stroke with oxygen atoms and molecules produces ozone in the same manner as that photo­ chemically produced above the tropopause. The seco"nd involves the photochemical action of the light of the lightning stroke with nitrogen dioxides. Nitrogen oxides, part of which are dioxides, are produced when the lightning discharge causes nitrogen of the atmo­ sphere to unite with oxygen. Haagen-Smit (1952, 1956, 1958) has established that nitrogen dioxide, as well as hydrocarbons, in the presence of ultraviolet light is a producer of ozone. Therefore, the nitrogen dioxide resulting from one lightning stroke becomes the sour,ce of ozone production when exposed to the ultraviolet light component of subsequent lightning strokes. The amount of ozone produced by the lightning stroke is questionable. The observations indicate that it must be rather insignificant. If the lightning stroke were the primary source, or the sole source, of ozone associated with thunderstorms as most literature implies, significant increases of ozone should occur with all thunderstorms having heavy lightning activity and sufficient downdraft to transport the ozone to the sur­ face, regardless of the location or the time of year. Hundreds of thunderstorms occurred within 20 miles of the observation station during the past 10 years. No increase, or a de­ crease, in ambient ozone occurred in many instances regardless of the amount and intensity of lightning. An example was on June 23, 1967, when thunderstorms began near 1: 00 PM. Prior to the start of the thunderstorm activity, the ozone meter had been recording from 2 to 2.2 pphm for 2 Yz hours. At the beginning of the first thunderstorm, the concentration dropped 10-20 percent. Thunder­ storms continued for 4 hours during which time the sferics counters indicated better than 4,700 lightning strokes. One thousand and six hundred were recorded in 1 hour. Concentrations never reached levels above 2 pphm during the remainder of the day. It occurs to 508 INFLUENCE OF THUNDERSTORMS ON ENVIRONMENTAL OZONE the author that there should have been some increase if the lightning produced ozone were significant. Examination of records showed that concentrations frequently increased 1 to 4 pphm with thunderstorms during the late winter, spring, and early summer months. Occasionally the increase was in excess of 4 pphm. Except for a very few instances, concentrations changed little, if at all, during thunderstorms of late summer and early fall, irrespective of the lightning activity of the storm. The question then arises as to why there are ozone increases with some storms and not with others. To resolve the question it is necessary to first consider the likely source of the ozone. Secondly, it will be necessary to review the aspects of the thunderstorm which might transport ozone into the environment of the lower atmosphere. It was previously noted that most ozone is located above the tro­ popause. The height of the tropopause varies through the year. It is normally lowest in the late winter, spring, and early summer. This is the time of year that the total amount of ozone in the northern hemisphere is the greatest (Brewer, 1960). There is normally little transport of matter, including ozone, across the tropopause. With some of the extratropical disturbances including thunderstorms, how­ ever, stratospheric matter is able to move into the troposphere (Staley, 1960, Danielsen, 1964). It is well known today that thunder­ storms penetrate the tropopause. Long (1966), in a 4 year study of thunderstorms, found a large number of cumulonimbus clouds with tops of 60,000 feet, or higher, from the Gulf of Mexico northward. Along the northern Gulf coast the maximum number of penetrations of the tropopause occurred in the spring. The National Weather Service radar located at Apalachicola, Florida, often spots thunder­ storms over north Florida with tops well above the tropopause. This is, in fact, one of the criteria used by the National Weather Service to identify storms with sufficient development to be severe with hail, high winds, and tornadoes.